After having been created by suitable forming steps, either by hot-rolling or cold-rolling, low-alloy sheet steel for automobile body construction is not corrosion-resistant. This means that oxidation occurs on the surface already after a relatively short time because of humidity in the air.
It is known to protect sheet steel against corrosion by means of appropriate corrosion-protection layers. In accordance with DIN-50900, Part 1, corrosion is defined as the reaction of a metallic material with its surroundings, which causes a measurable change in the material and can lead to a degradation of the function of a metallic structural part or an entire system. To prevent corrosion damage, steel is customarily protected, so that it withstands corrosion damage during the required period of use. The prevention of corrosion damage can take place by affecting the properties of the reaction partners and/or by changing the reaction conditions, separation of the metallic material from the corrosive medium by means of applied protective layers, as well as by electro-chemical steps.
In accordance with DIN 50902, a corrosion-protection layer is a layer produced on a metal, or in the area of the surface of a metal, which consists of one or several layers. Multi-layered coatings are also called corrosion-protection systems.
Possible corrosion-protection layers are, for example, organic coatings, inorganic coatings and metallic coverings. The purpose of metallic corrosion-protection layers lies in transferring the properties of the applied material to the steel surface for as long as possible a time. Accordingly, the selection of an effective metallic corrosion protection presupposes the knowledge of the corrosion-chemical connections in the system of steel/coating material/corrosive medium.
The coating metals can be electro-chemically nobler, or electro-chemically less noble in comparison with steel. In the first case the respective coating metal protects the steel by the formation of protective layers alone. This is referred to as so-called barrier protection. As soon as the surface of the coating metal has pores or is damaged, a “local element” is formed in the presence of moisture, in which the base partner, i.e. the metal to be protected, is attacked. Among the nobler coating metals are tin, nickel and copper.
On the one hand, base metals produce protective covering layers, on the other hand they are additionally attacked in case of leaks in the layers, since in comparison with steel they are more base. In case of damage to such a coating layer, the steel is accordingly not attacked, instead first the baser coating metal corrodes because of the formation of local elements. This is referred to as a so-called galvanic or cathodic corrosion protection. Zinc, for example, is among the baser metals.
Metallic protective layers are applied in accordance with various methods. Depending on the metal and the method, the connection with the steel surface is of a chemical, physical or mechanical type and extends from alloy formation and diffusion to adhesion and mere mechanical cramping.
The metallic coatings should have technological and mechanical properties similar to steel and should also behave similar to steel in connection with mechanical stresses or plastic deformations. Accordingly, the coatings should not be damaged in the course of forming and should not be negatively affected by forming processes.
When applying hot-dip galvanizing coatings, the metal to be protected is immersed in liquid metallic melts. Appropriate alloy layers are formed at the phase boundary between steel and the coating metal by dipping in the melt. An example of this is hot-dip galvanizing.
In the course of hot-dip galvanizing, the steel tape is conducted through a zinc bath, wherein the zinc bath has a temperature of roughly 450° C. Hot-dip galvanized products show great corrosion resistance, are well suited to welding and forming, their main areas of use are in the construction, automobile and home appliance industries.
Moreover, the creation of a coating from a zinc-iron alloy is known. To this end, following hot-dip galvanizing these products are subjected to diffusion-annealing at temperatures above the melting point of zinc, generally between 480° C. and 550° C. In the process, the zinc-iron alloy layers grow and eat up the zinc layer above. This method is called “galvannealing”. The zinc-iron alloy created in this way also has a high corrosion resistance, is well suited to welding and forming. Main areas of use are the automobile and household appliance industries. By dipping into a melt it is moreover also possible to produce other coatings from aluminum-silicon, zinc-aluminum and aluminum zinc.
The production of electrolytically-deposited metal coatings is furthermore known, i.e. the electrolytic deposition of metal coatings from electrolytes taking place by means of the passage of electrical current.
Electrolytic coating is also possible in connection with those metals which cannot be coated by means of the hot-dip galvanizing method. With electrolytic coating, customary layer thicknesses mainly lie between 2.5 and 10 μm, they are therefore thinner than coatings produced by the hot-dip galvanizing method. Some metals, for example zinc, also permit thick-film coatings in case of electrolytic coating. Electrolytically zinc-coated metal sheets are primarily employed in the automobile industry, because of the great surface quality, these metal sheets are mainly employed in the area of the outer skin. They are easy to form, are suitable for welding and have a good storage capability, as well as surfaces which are easy to paint and are matte.
In automobile construction in particular, efforts are being made to make the body continuously lighter. This is connected on the one hand with the fact that lighter vehicles use less fuel, on the other hand vehicles are more and more equipped with additional functions and additional units, which entails an increase in weight, which is intended to be compensated by a lighter shell.
However, at the same time the requirements made on safety of motor vehicles are increasing, wherein the body is responsible for the safety of the people in a motor vehicle and their protection in case of accidents. Accordingly, in connection with lighter gross weight of the body there is the requirement for providing increased safety in case of accidents. This is possible only by employing materials of increased sturdiness, in particular in the area of the passenger compartment.
In order to obtain the required sturdiness it is necessary to use types of steel with increased mechanical properties, or to treat the types of steel used in such a way that they have the required properties.
For providing sheet steel with increased sturdiness it is known to form structural steel parts in one step and to harden them at the same time. This method is also called “press hardening”. In the course of this a piece of sheet steel is heated to a temperature above the austenizing temperature, customarily above 900° C., and is subsequently formed in a cold tool. In the process the tool forms the hot piece of sheet steel which, because of its surface contact with the cold mold, is very rapidly cooled, so that the per se known hardening effects in connection with steel occur. It is furthermore known to first form the sheet steel and subsequently to cool the formed structural sheet steel part in a calibrating press and to harden it. In contrast to the first method it is advantageous here that the sheet metal is formed in the cold state and more complex shapes can be obtained in this way. However, in connection with both methods the sheet metal surface is oxidized by the heating, so that the surface of the sheet metal must be cleaned after forming and hardening, for example by sandblasting. The sheet metal is subsequently cut, and possibly required holes are punched out. In the course of this it is disadvantageous that the sheets have a large hardness during mechanical processing and therefore processing becomes expensive and a large amount of tool wear occurs in particular.
The aim of U.S. Pat. No. 6,564,604 B2 is to make available pieces of sheet steel which are subsequently subjected to heat treatment, as well as making available a method for producing parts by press-hardening these coated pieces of sheet steel. It is intended here to assure in spite of the temperature increase that the sheet steel does not decarbonize and the surface of the sheet steel does not oxidize prior to, during and after hot-pressing or the heat treatment. To this end it is intended to apply an alloyed inter-metallic mixture to the surface prior to or following stamping, which is intended to provide protection against corrosion and decarbonization and in addition can provide a lubrication function. In one embodiment this publication proposes the use of a customary, apparently electrolytically applied zinc layer, wherein this zinc layer is intended to be converted into a homogeneous Zn—Fe-alloy layer together with the steel substrate during a subsequent austenization of the sheet metal substrate. This homogeneous layer structure is verified by means of microscopic photos. In contrast to earlier assumptions, this coating is said to have a mechanical resistance capability which protects it against melting. However, such an effect is not shown in actual use. In addition, the use of zinc or zinc alloys is intended to offer a cathodic protection of the edges if cuts are being made. However, it is disadvantageous in connection with this embodiment that with such a coating—contrary to the statements in this publication—there is hardly any corrosion protection of the edges and, if this layer is damaged, only a poor corrosion protection is achieved in the area of the sheet surface.
A coating is disclosed in the second example of U.S. Pat. No. 6,564,604 B2, 50% to 55% of which consist of aluminum, 45% to 55% of zinc, and possibly small amounts of silicon. Such a coating is not new per se and is known under the name Galvalume®. It is stated that the coating metals zinc and aluminum are said to form, together with iron, a homogeneous zinc-aluminum-iron alloy coating. In connection with this coating it is disadvantageous that a sufficient cathodic corrosion protection is no longer achieved by means of it, but in connection with its use in the press-hardening method the predominant barrier protection achieved with it is not sufficient, since damage to partial areas of the surface is unavoidable. In summary it can be stated that the method described in this publication is not capable of solving the problem that generally cathodic corrosion layers on the basis of zinc are not suitable for protecting sheet steel which is intended to be subjected to heat treatment following coating, and are moreover subjected to a further shaping or forming step.
A method for producing a structural sheet metal part is known from EP 1 013 785 A1, wherein the sheet metal is said to have an aluminum layer or an aluminum alloy layer on its surface. A structural sheet metal part provided with such coatings is intended to be subjected to a press-hardening process, wherein an alloy with 9 to 10% silicon, 2 to 3.5% iron, the remainder aluminum with impurities, and a second alloy with 2 to 4% iron and the remainder aluminum with impurities, are cited as possible coating alloys. Such coatings are known per se and correspond to the coating of hot-dip-aluminized sheet steel. The disadvantage in connection with such a coating is that only a so-called barrier protection is achieved. In the instant such a protective barrier coating is damaged, or in case of cracks in the Fe—Al layer, the base material, in this case the steel, is attacked and corrodes. No cathodic protective effects are provided.
Moreover, it is disadvantageous that in the course of heating the sheet steel to the austenizing temperature and the subsequent press-hardening step, such a hot-dip-aluminized coating is chemically and mechanically stressed to such an extent that the finished structural part does not have a sufficient corrosion-protective layer. As a result it can therefore be stated that such a hot-dip-aluminized coating is not well suited to press-hardening into complex geometrical shapes, i.e. for heating sheet steel to a temperature which lies above the austenizing temperature.
A method for producing a coated structural part for vehicle production is known from DE 102 46 614 A1. This method is intended for solving the problems of the previously mentioned European Patent Application 1 013 785 A1. In particular, it is stated that in accordance with the dipping process of European Patent Application 1 013 785 A1 an inter-metallic phase is said to already be formed in the course of coating the steel, wherein this alloy layer between the steel and the actual coating is said to be hard and brittle and to tear during cold-forming. Because of this, micro-cracks are said to be formed up to such a degree that the coating itself is detached from the basic material and in this way loses its protective effects. Therefore DE 102 46 614 A1 proposes to apply a coating in the form of metal or a metal alloy by means of a galvanic coating method in an organic, non-aqueous solution, wherein aluminum or an aluminum alloy is said to be a particularly well suited, and therefore preferred coating material. Alternatively zinc or zinc alloys would also be suitable. Sheet metal coated in this way can subsequently be preformed cold and finished hot. However, with this method the disadvantage is that an aluminum coating, even if applied electrolytically, no longer offers corrosion protection in case of damage to the surface of the finished structural part, since the protective barrier was breached. In connection with an electrolytically deposited zinc coating it is disadvantageous that the greater portion of the zinc oxidizes during heating for heat forming and is no longer available for cathodic protection. The zinc evaporates in the protective gas atmosphere.
A method for producing metallic profiled structural parts for motor vehicles is known from DE 101 20 063 C2. In connection with this method for producing structural metallic profiled parts for motor vehicles, a starting material made available in the form of tape is fed to a roller profiling unit and is formed into a rolled profiled section. Following the exit from the roller profiling unit it is intended to heat at least partial areas of the rolled profiled section inductively to a temperature required for hardening and to subsequently quench them in a cooling unit. Thereafter the rolled profiled sections are cut into the profiled structural parts. A particular advantage of roller profiling is said to lie in the low manufacturing costs because of the high processing speed, and tool costs which are lower in comparison with a pressing tool. A defined tempered steel is used for the profiled structural part. In accordance with an alternate of this method it is also possible to inductively heat partial areas of the starting material prior to their entry into the roller profiling unit to the temperature required for hardening and to quench it in a cooling unit prior to cutting off the rolled profiled sections. In connection with the second alternative it is disadvantageous that cutting to size must take place already in the hardened state, which is problematical because of the great hardness of the material. It is furthermore disadvantageous that in the already described prior art the profiled structural parts cut to size must be cleaned, or descaled, and that a corrosion-protection coating must be applied after descaling, wherein such corrosion-protection coatings customarily do not provide a very good cathodic corrosion protection.